Haem oxygenase (HO‐1) plays a key role in the generation of cellular CO and is dysregulated in a wide variety of cancers.[22] Hypoxic conditions can cause an increase in expression of HO‐1,[23] thus probe 4 was tested against CO produced in hypoxia (24 h), in MCF‐7 cells, that were treated with 4, then fixed to maintain the induced effect and compared to cells grown in a normoxia environment. Here, HMOX1 is linked to cancer.